Tub grinders have been widely used for reducing the particle size of bulk materials. This type of apparatus includes a large tub having an open top for receiving bulk material to be processed. A stationary floor in the tub is in a generally-horizontal plane. A rotor member is mounted under the floor, and hammers or blades on the rotor extend into the tub through an opening in the stationary floor. The rotor is rotated at a high rate of speed, and the tub is rotated slowly. The rotor must be precision-built and balanced so as to eliminate vibration problems.
The hammers or blades on the rotor strike the bulk material and grind it into smaller particles. A screen having small apertures in it is positioned in close proximity to the rotor under the floor. The bulk material cannot pass through the screen until the particles are smaller than the apertures.
As the tub is rotated, the bulk material is continually urged against the rapidly-revolving rotor member where the high speed hammers strike the bulk material. The hammers are usually made from a piece of flat steel about 4 to 6 inches long with a hole punched through each end. The hammers are normally heat-treated, and the wear tip is surfaced with a hardened material to reduce the effect of the high-speed wear to which they are subjected, since the corners of the hammers wear off rapidly during high-speed impact with the bulk material. The hammers must be turned end-for-end or replaced often, since they lose their ability to push the material through the screens after the corners are worn down.
The processed bulk material either falls through the screen system and is carried out by conveying or it is carried around the rotor by the hammers and thrown out the back side of the rotor and back into the tub where it is pushed around again by the rotating tub. Then the process is repeated.
The rotor in a conventional tub grinder typically is rotated at a speed of about 1800 to 2500 rpm. Because tub grinders usually are powered by a farm tractor whose power take-off shaft turns at either 540 or 1000 rpm, a gear box or belt drive mechanism is needed to increase the rotor speed to the required rpm. This high rate of speed is necessary to produce sufficient centrifugal force to keep the short lightweight hammer extended and to keep the machine operating properly. When this type of machine is operated at a lower rate of speed, there is insufficient centrifugal force to keep the hammers extended when they contact the bulk material, whereby hammer fly-back occurs which reduces machine performance and can cause damage to the machine itself.
The conventional tub grinder utilizes a hammer tip speed of 780 to 900 feet per second at 1000 rpm PTO speed. Such grinders were originally designed to pulverize grains, bones and other hard brittle substances. When these machines are used to grind forage materials such as hay, they consume a large amount of power and create a significant amount of fine, powdered material. This dust makes the machine very unpleasant to operate. Also, the dust in the atmosphere creates pollution problems and wastes feed.
The leaves of the forage material contain the most nutrients. They are also the most fragile and they are easily pulverized into dust during the grinding process. When this dust is lost the overall quality of the forage is drastically reduced. When the ground forage is handled or exposed to windy conditions this loss continues until the ground forage product is consumed. Large ruminant animals (such as milk cows) need more roughage in their feed. If the feed has too many "fines", i.e., small particles of feed, health problems result for the cows which reduces milk production and animal value.
An improved tub grinder is described in my U.S. Pat. No. 5,207,391, incorporated herein by reference, which eliminates the need for a screen in tub grinding apparatus.
There has not heretofore been described tub grinding apparatus having the features and advantages provided by the present invention.